I'd be interested in something like this. I only use the Overground however and something tells me that any app, official or otherwise, would omit that. When I get my Desire a National Rail app and an Overground app would be a brilliant combo for me.
http://www.androidzoom.com/android_applications/travel/london-journey_fzjg.html
I can't quite remember if it covered overground but I used this app for the underground and it was really good. You get 24 hour refund so you might as well try it.